<?php 
$dom = new DOMDocument("1.0", "ISO-8859-1"); //versao 1.0 com encoding ISO-8859-1 
$dom->preserveWhiteSpace = false; //descartar espacos em branco
$dom->formatOutput = true; //setar codigo legivel 

$dom->load("usuarios.xml");

$params = $dom->getElementsByTagName('usuario');

foreach ($params as $param) {
      echo $param -> getAttribute('login').'.<br>';
}
?>